This invention is generally directed to DC and low frequency AC isolation of shielded conductors. It is particularly directed to AC line isolation of coaxial cables of the type used to couple an antenna to the tuner of a television receiver.
Some television manufacturers currently sell receivers having a so-called "hot chassis", meaning that the receiver's chassis may be at the potential of the AC power line. For consumer protection, various components of the receiver, such as the tuner, are isolated from the hot chassis. However, it is desirable to include an additional level of AC isolation, especially for hot chassis receivers. The function of the extra level of isolation is to further isolate the tuner input from antenna connectors usually located on the rear panel of the receiver. Thus, should the first level of isolation fail, a consumer would still not receive an electrical shock when connecting an antenna or cable lead to the connectors on the receiver's rear panel.
A conventional approach to providing the second level of isolation is to couple the connectors on the receiver's rear panel to the tuner via a shielded cable whose inner and outer conductors are incapable of passing the AC line frequency. Typically, this cable may be in two parts and joined together capacitively so that low frequency AC cannot pass. The point at which the capacitive coupling is effected is typically housed in a metal box to prevent ambient radiation from coupling to the desired television field within the cable. This approach is described more fully in the discussion of prior art found in U.S. application Ser. No. 184,720, filed Sept. 8, 1980.
Another approach to second level isolation is disclosed in U.S. application Ser. No. 282,824 filed July 13, 1981 which is a continuation-in-part of U.S. application Ser. No. 132,020 filed Mar. 20, 1980, abandoned. Briefly, the latter approach AC isolates the shield of a cable between the connectors on the receiver's rear panel and the tuner by including a ceramic capacitor in the cable's shield connection to the tuner. It avoids the need for a metal box around this connection by the inclusion of ferrite material for absorbing ambient radiation. However, this approach does not provide AC isolation for the cable's center conductor.
Accordingly, it is an object of the present invention to provide an improved AC line isolator.
It is a more specific object of the invention to provide a DC and AC line isolator which isolates the center conductor of a coaxial cable but which does not require a metal box to shield the conductor from ambient radiation.